The present invention relates to high surface area metal material and high capacitance electrolytic capacitors made therefrom. More particularly, the present invention relates to a porous metallic columnar microstructure formed on a substrate, and electrolytic capacitors using the microstructure, as well as methods of making the microstructure and the capacitors.
Electrolytic capacitors, made from metal material, have been a major contributor to the miniaturization of electronic circuits and have made possible the application of such circuits in extreme environments. Metallic capacitors typically are manufactured by compressing metal material, e.g., powder, to form a pellet, sintering the pellet in a furnace to form a porous metallic body (anode), and then subjecting the sintered anode to anodization in a suitable electrolyte to form a continuous dielectric oxide film on the sintered anode. The anode is then immersed in an aqueous solution of manganese nitrate and then heated to produce an anode having a manganese dioxide coating on its surface.
The performance characteristics of capacitors or electrodes formed from capacitor grade materials are expressed in terms of specific charge and electrical current leakage. The specific charge is a measure of electrical charge capacity of the capacitor and is usually proportional to the surface area of the metal material as a sintered and anodized pellet. The electrical leakage is an indication of how well the capacitor holds the specific charge. Capacitors with improved electrical leakage characteristics are recognized as having higher reliability.
Development of metal materials suitable for making metallic capacitors has resulted from efforts by both capacitor producers and metal processors to delineate the characteristics required for metal material to best serve in the production of quality capacitors. Such characteristics include specific surface area, purity, shrinkage, pressability, and the like. With respect to surface area, the metal material should provide an adequate electrode surface area when formed into a porous body and thereafter sintered. The μFV/g of metallic capacitors can be related to the specific surface area of the sintered porous body produced by sintering a metal pellet. The specific surface area of metal material can be related to the maximum μFV/g attainable in the sintered porous body. High sintering temperatures tend to shrink the porous body, thus reducing its net specific surface area and the capacitance of the resulting capacitor. Minimizing the loss of specific surface area under sintering conditions, i.e., shrinkage, is necessary to produce high μFV/g metallic capacitors.
As discussed above, the μFV/g of a metal pellet can be a function of the specific surface area of the sintered metal material. Greater net surface area can be achieved, of course, by increasing the quantity (grams) of metal per pellet; but, cost and size considerations have dictated that development be focused on means to increase the specific area of metal material, that is, to increase volumetric efficiency. Thus, conventional production of metallic capacitor anodes having a given capacitance has required the use of metal powders having specified properties, e.g., a certain surface area, for an entire production lot of anodes.
Accordingly, a need exists for a method to produce a high surface area metal material for use in forming capacitors having high capacitance. Additionally, a need exists for a method of forming an electrolytic capacitor anode without the costs and surface area reduction associated with sintering the metal material. A further need exists for a method of forming a capacitor anode in which the capacitance of the anode can be controlled by varying any one of a number of factors during production of an anode.